1. Field of the Invention
The present invention relates to an optical disk capable of high density recording utilizing a rewritable phase-change medium. Particularly, it relates to a phase-change medium whereby groove signals compatible with CD (compact disk) standards, can be obtained in spite of low reflectance, and deterioration during repeated overwriting of data is little, while maintaining high contrast.
2. Discussion of the Background
Along with an increasing amount of information in recent years, a recording medium capable of recording and retrieving a large amount of data at a high density and at a high speed has been demanded, and an optical disk is expected to be just suitable for such an application.
Optical disks include a write-once type disk capable of recording only for once and a rewritable-type disk capable of recording and erasing many times.
As the rewritable-type optical disk, a magneto-optical medium utilizing a magneto-optical effect, or a phase-change medium utilizing the change in reflectance due to the reversible change in the crystal state, may be mentioned.
The phase-change medium has a merit that it is capable of recording/erasing simply by modulating the power of a laser beam without requiring an external magnetic field, and the size of a recording and retrieving device can be made small.
Further, it has a merit that a high density recording can be attained by a shorter wavelength light source without any particular alteration of the material of e.g. the recording layer from the currently predominantly employed medium capable of recording and erasing at a wavelength of about 800 nm.
As the material for the recording layer of such a phase-change medium, a thin film of a chalcogenic alloy is often used. For example, an alloy of GeSbTe type, InSbTe type, GeSnTe type or AgInSbTe type may be mentioned.
In a rewritable phase-change type recording medium which is practically employed at present, an unrecorded or erased state is a crystalline state, and recording is carried out by forming an amorphous bit. The amorphous bit is formed by heating the recording layer to a temperature higher than the melting point, followed by quenching. To prevent evaporation or deformation of the recording layer by such heat treatment, it is common to sandwich the recording layer with heat resistant and chemically stable dielectric protective layers. In the recording step, these protective layers facilitate heat dissipation from the recording layer to realize overcooled state, and thus contribute to formation of the amorphous bit.
Further, it is common that a metal reflective layer is formed on the above described sandwich structure to obtain a quadri-layer structure, whereby the heat dissipation is further facilitated so that the amorphous bit will be formed under a stabilized condition.
Erasing (crystallization) is carried out by heating the recording layer to a temperature higher than the crystallization temperature and lower than the melting point of the recording layer. In this case, the above-mentioned dielectric protective layers serve as heat accumulating layers for keeping the recording layer at a temperature sufficiently high for solid phase crystallization.
A phase-change medium which is capable of carrying out the erasing and rewriting steps solely by intensity modulation of one focused light beam, is called a 1-beam overwritable phase-change medium (Jpn. J. Appl. Phys., 26 (1987), suppl. 27-4, pp. 61-66).
With a recording system employing the 1-beam overwritable phase-change medium, the multilayer structure of the recording medium and the circuit structure of the drive can be simplified. Therefore, an attention has been drawn to this system as an inexpensive high density and large capacity recording system.
In recent years, rewritable compact disks (CD-Rewritable, CD-RW) have been proposed ("CD-ROM professional", USA, September, 1996, p. 29-44, or preparatory papers for Phase-change Optical Recording Symposium, 1995, p. 41-45).
With CD, rows of pits with length modulated by data sequence, formed on a substrate with a pitch of 1.6.+-.0.1 .mu.m are scanned by a focused laser beam having a wavelength of 780.+-.30 nm from the rear side of the substrate to read out the recorded information. Here, the reflectance at a non-pitted portion is stipulated to be at least 70%.
With CD-RW, it is difficult to accomplish compatibility with CD if such a high reflectance as at least 70% is included. However, by bringing the reflectance at an unrecorded portion to a level of from 15 to 25% and the reflectance at a recorded portion to a level of less than 10%, compatibility with CD can be secured with respect to the record signals and groove signals, and by adding an amplifying system to cover the low reflectance to the retrieving system, compatibility can be secured within the range of the existing CD drive technology.
In CD-RW, grooves are used as recording tracks, and recording is carried out in the grooves, and wobbling is used for these grooves to include address information (JP-A-5-210849).
FIG. 1 shows a schematic view illustrating wobbling grooves 2 formed on the surface of a substrate 1. However, the wobble amplitude is exaggerated. The wobbling is called "wobble" and frequency-modulated (FM) by a carrier frequency of 22.05 kHz, and its amplitude (Wobble Amplitude) is very small at a level of 30 nm as compared with the pitch of grooves 2 (i.e. the distance between the imaginary center lines of grooves 2: usually about 1.6.+-.0.1 .mu.m).
Such a wobble frequency-modulated by absolute time information or address information, is called ATIP (Absolute Time In Pre-groove) or ADIP (Address In Pre-groove), which has already been used in a recordable compact disk (CD-Recordable, CD-R) or in a mini disk ("CD family", coauthored by Heitaro Nakajima, Takao Inohashi and Hiroshi Ogawa, Ohm-sha (1996) chapter 4 and Proceedings of the IEEE, vol. 82 (1994) p. 1490).
The recording process of the above phase-change medium involves a drastic heat cycle such that the recording layer is melted and then quenched to a temperature lower than the melting point within a few tens nano seconds. Therefore, even if the recording layer is sandwiched by dielectric protective layers, microscopic deformations and segregations will be accumulated by repetitive overwriting for a few thousands to a few tens thousands times, and will eventually lead to an increase of optically recognizable noises or formation of local defects of micron order (J. Appl. Phys., 78 (1995), pp 6980-6988).
Substantial improvements have been made by modifying the materials for the recording layer and the protective layers, or the multilayer structure. However, there is essentially an upper limit in the number of rewritable times, and it is usually smaller by at least one figure than a usual magnetic recording medium or magneto-optical recording medium.
With the above-described CD-RW, recording is carried out at a low linear velocity at a level of at most 6 times of the CD linear velocity and under such a severe condition as mark length modulation recording, and a higher level of repetitive overwriting durability is required.
Further, in the mark length modulation recording employing a phase-change medium for forming amorphous marks while an unrecorded state is a crystalline state, it is desired that the outlines of the amorphous marks are smooth and distinct. For this reason, in place of a conventional GeTe-Sb.sub.2 Te.sub.3 pseudo binary alloy, a material for a recording layer having a smaller grain size, is desired.
Further, from the study by the present inventors, it has been found that the groove geometry is required to secure the compatibility of groove signals with the CD standards, rather lowers the repetitive overwriting durability of the phase-change medium. Namely, within a range of the groove geometry (depth: 20 to 100 nm, width: 0.2 to 0.8 .mu.m) where there will be no trouble in tracking servo (a push-pull method or a 3 beam method) with a focused light having a wavelength of 780 nm.+-.30 nm, the groove depth is required to be less than 60 nm, and the groove width is required to be within a range of from 0.3 to 0.6 .mu.m in order to bring push-pull signals after recording to the same level as ROM standards (about 0.04 to 0.09) to secure the compatibility with CD-ROM (JP-A-8-21550, but this patent concerns nothing about the repetitive overwriting durability). This relation is a parameter determined substantially solely by the groove geometry, which does not substantially depend on the multilayer structure of the phase-change medium.
On the other hand, there is a tendency that the repetitive overwriting durability is better when the groove is deep and narrow in width. From the study by the present inventors, it has been found that the repetitive overwriting durability abruptly deteriorates when the groove depth becomes shallower than 50 nm.
Thus, to secure the compatibility with the groove signals of conventional CD, the overwriting durability has to be sacrificed to some extent, but it is desired to minimize such a sacrifice.
On the other hand, in addition to the above described restriction derived from the groove signals, a new phenomenon for deterioration has been found with a CD-RW medium employing a phase-change medium, such that wobble signals are likely to leak into recorded signals by repetitive overwriting. The wobble is essential also to CD-RW to impart address information essential to detect an unrecorded region where information is to be recorded. If it is attempted to reduce the groove width to overcome the deterioration of the overwriting durability due to reduction of the groove depth, the groove walls tend to be damaged by the heat by a recording light beam edge, whereby deterioration of the signals attributable to the wobble signals is believed to be accelerated.
Further, the groove bottom also undergoes deformation by the heat generation of the recording layer. The lower protective layer has a function of not only suppressing the temperature rise of the substrate surface by the heat insulating effect but also mechanically suppressing the deformation of the substrate. Accordingly, a ZnS-SiO.sub.2 mixture film or the like is widely used from the viewpoint of the thermal conductivity and mechanical properties.
From the study by the present inventors, it has been found very difficult to satisfy both the productivity and the repetitive overwriting durability because of the restriction of the thickness of the lower protective layer due to the optical requirements for the compatibility with the CD standards. Due to such an additional condition required to secure the compatibility with the CD standards, the number of repetitive overwriting further decreases by at least one figure to a level of a few thousands times.
The method of rewriting information per a sector unit as in a magneto-optical disk, has not yet been established with CD-RW. However, if such a method will be practically used, it is likely that the number of rewriting to a specific sector will exceed 1,000 times, whereby the problem of deterioration due to repetitive overwriting will be more serious.
Accordingly, it has been an acute demand to improve the repetitive overwriting durability while securing the compatibility with the current CD standards as far as possible.